1. Field of the Invention
The present invention relates to an electrical double layer capacitor and more particularly to the electrical double layer capacitor that can be suitably used when energy density per unit volume is made higher.
The present application claims priority of Japanese Patent Application No. 2004-238394 filed on Aug. 18, 2004, which is hereby incorporated by reference.
2. Description of the Related Art
An electrical double layer capacitor has advantages that cannot be provided by a nickel-hydrogen secondary cell, lithium-ion secondary cell or a like; that is, the electrical double layer capacitor can be charged rapidly and discharged at a large electric current and its characteristics are not degraded even when charging and discharging are repeated ten thousand times or more. Due to this, in recent years, expectations of the electrical double layer capacitor that can be used as an alternative to secondary cells or as an auxiliary power supplying source are expanding. In the electrical double layer capacitor, a capacitor element is formed which is made up of a pair of polarized electrodes each having polarized electrode layers containing activated carbon as a main ingredient and each electrode facing another electrode with a separator being interposed between positive and negatively polarized electrodes and the polarized electrode layers making up the electrodes are impregnated with an electrolytic solution. Then, an electrical double layer is formed at an interface between each of the polarized electrode layers and the electrolytic solution. When a voltage is applied to the electrical double layer capacitor, electric charges are accumulated in an electrostatic capacitor of the electrical double layer.
Such a conventional electrical double layer as above is disclosed in, for example, Japanese Patent Application Laid-open No. 2002-83749 (see its Abstract and FIG. 1). In the disclosed conventional electrical double layer capacitor, a pair of polarized electrodes each having electrode layers being supported on an aluminum foil power collecting body and each facing another is mounted with a separator being interposed between the polarized electrodes, and each of capacitor elements being wound in a vortex form is housed, together with the electrolytic solution, in a metal case being sealed by a seal.
In recent years, it is required that the electrical double layer capacitor has a high energy density. To achieve this, the electrical double layer capacitor 1 of an electrode winding-around type, as shown in FIGS. 9 and 10, is so configured that a positive electrode 4 in which positive electrode layers 3 are attached, in a covered manner, to surfaces of both sides of a power collecting body 2 made of an aluminum foil and a negative electrode 7 in which negative electrode layers 6 are attached, in a covered manner, to surfaces of both sides of a power collecting body 5 wound with a separator 8 being interposed between the positive electrode 4 and negative electrode 7. By configuring as above, it is possible to make smaller a shape of the electrical double layer and to improve an energy density per unit volume of the electrode. In this case, it is necessary that the negative electrode 7 covers the positive electrode 4 necessarily. This is because there is a possibility that a decomposition reaction of electrolytic solution components in the positive electrode 4 not facing the negative electrode 7 occurs and gas is generated within the electrical double layer capacitor, and an internal pressure is raised, as a result, causing rupture. For example, in an electrical double layer capacitor disclosed in Japanese Patent Application Laid-open No. 2003-100569 (see Abstract and FIG. 1), by making a width of the negative electrode be larger than that of the positive electrode, the negative electrode covers the positive electrode necessarily. By configuring as such, reliability of the electrical double layer capacitor can be ensured.
However, the conventional electrical double layer capacitor has following problems. That is, in the electrical double layer capacitor shown in FIG. 10, the negative electrode 7 has to be placed so as to cover the positive electrode 4 necessarily and, therefore, the negative electrode 7 is excessively wound when the winding of the negative electrode 7 is started or ended. As a result, the shape of the electrical double layer capacitor 1 becomes larger and the excessively wound negative electrode 7 does not serve to make higher electrostatic capacitance and, therefore, it is impossible to make higher energy density per unit volume of the electrical double layer capacitor.